The invention relates to a vehicle steering system with a device for changing the gear ratio, and with an electrical auxiliary drive.
A series of vehicle steering systems is known, for which the function of rotational speed conversion and an auxiliary drive are realized in separate devices.
Accordingly, in the DE 19823721 A1, a rotational speed superimposition is introduced. A housing, in which there are gearings of an internal gear wheel of two planetary gear trains, is driven here by a driving wheel. An electric motor, disposed in the housing, drives the sun wheel of the second planetary gear train. The planet carrier of the first planetary gear train drives the sun wheel of the second planetary gear train. The planet gears of the second planetary gear train are supported at the internal gear wheel of the housing and the planet carrier is connected with the transmission output shaft. In the embodiments shown, the driving mechanism of the sun wheel of the first planetary gear train is realized directly by the rotor of the electric motor. The desired rotational speed gear ratios can be represented by an appropriate control of the electric motor. However, this solution, shown in the state of the art, has some decisive disadvantages. When the steering wheel is turned, the whole unit is turned along with it. Therefore, when turning the steering wheel, the driver must employ the whole driving torque for turning the steering pinion and, additionally, overcome the inertia of the whole unit. In order to equalize this, such systems are equipped with an additional auxiliary power support at a different place. In addition, such a transmission unit is afflicted with play. As a result, the driving shaft and driven shaft must be mounted, so that the whole unit is held and a connection with a high positional stability between the steering wheel and the driven shaft is attained. This represents a not inconsiderable structural expense. Furthermore, the coupling of electric energy into the electric motor, which rotates along with the steering wheel, is expensive.
In a further state of the art, the DE 19852447 A1, a solution for the rotational speed conversion is introduced, for which the electric motor is coupled over a worm drive with the speed-changing transmission constructed as a planetary gear train. The transmission unit is fixed to the car body here, so that the driver does not have to support the whole of the torque, which is introduced by the electric motor. However, a series of disadvantages is also associated with this solution. The coupling-in over a worm transmission leads to very low efficiencies in the rotary speed conversion. Furthermore, the arrangement requires appreciable space, which, due to the geometrically determined positions of the components with respect to one another, is not very flexible. Here also, an additional auxiliary power support is required at a different place.
Moreover, the above-mentioned state of the art jointly has even further disadvantages. All steering systems require the highest degree of safety in the event of a failure of the electrical components. For example, it must be possible to steer the vehicle even if the electric motors fail. For the state of the art shown above, this means that, in the event of a power failure or other disorder, the torque, introduced by the steering wheel, must not be introduced into the electric motor. For this purpose, the transmissions with high conversions are designed with self-locking. However, this leads to low efficiencies and slow response times of the electric motor driving mechanisms. Independently of the change in the rotational speed conversion, an additional driving mechanism is required as power support for the steering (power steering).
It is an object of the invention to eliminate the disadvantages of the state of the art and, at the same time, to make a compact component available, the reaction force on the steering wheel not being increased noticeably if at all. At the same time, the system shall offer in a simple way the necessary redundancy in the event of a malfunction of the electrical units.
Pursuant to the invention, the whole of the steering device has only one electric motor, which, at the same time, introduces the energy for the desired rotational speed conversion and the auxiliary power support into the system. This inventive, new device, in which the change in the conversion ratio as well as the introduction of the auxiliary power is realized, is referred to in the following as steering differential. The torques from the steering wheel and from the electric motor are introduced into the steering differential and the whole of the torque is passed on to the steering adjustment. In so doing, it is unavoidable that the torque, introduced by the electric motor, must be supported at least partially at the torque, introduced by the steering wheel.
In contrast to the state of the art, however, the housing of the conversion ratio device is fastened to be body of the vehicle. However, by selecting suitable mechanical conversions in the superimposed gear mechanism between the driving mechanism of the electric motor, of the drive shaft driven by the steering wheel and the drive shaft, the torques, which become noticeable at the steering wheel and the torques, which are made available for adjusting the wheels, can be adjusted largely as desired to a fixed ratio to one another. In this connection, it should be noted that, for steering a vehicle, a driver requires a torque Ma. Due to this configuration, the number of components of the steering device is reduced significantly, because only one electric motor and, with that, only one crank mechanism is required in the steering device.
Pursuant to the invention, the electric motor, the drive shaft, which is connected non-rotationally with the steering wheel, and the driven device assigned to the wheels, such as a driven shaft or steering rack, are disposed coaxially with one another. The steering differential consists of two planetary gear trains, which are mounted in one housing and with the help of which the appropriate transmission conversions are realized. As a result, the construction becomes very compact.
In a special, preferred embodiment, a permanently energized synchronous motor is used as electric motor. For this electric motor, the stator with the energizing coils is connected permanently with the housing of the device and the rotor is disposed coaxially in the interior, surrounds the driven device and transfers its torque to the planet carrier of the first planetary gear train and, with that, causes the first planet carrier to rotate. By these means, the planet gear wheels are caused to rotate and, at the same time, are supported at the half of the internal gear wheel fastened to the housing. As a consequence, the torque is transferred to the second rotatable half of the internal gear wheel with a number of teeth different from that of the first internal gear wheel that is fastened to the housing. This second, rotatable half of the internal gear wheel is coupled non-rotationally with a first half of an internal gear wheel of the second planetary gear train. In this way, the torque is transferred from the first to the second planetary gear train. The torque is transferred from this half of the internal gear wheel to the planet gear of the second planet carrier. If the number of teeth of the first half of the internal gear wheel and the number of teeth of the second half of the internal gear wheel of the second planetary gear train are different, the torque is transferred to the second planet carrier. At the same time, the second planet carrier, in the coupled state, is connected non-rotationally with the drive shaft, which is connected non-rotationally with the steering wheel. By these means, the torque of the drive shaft is transferred to the second planet carrier. Moreover, the torques, introduced by the electric motor into the second planet carrier, are supported by the steering wheel.
The planet gears of the second planet carrier transfer the torque to the second half of the internal gear wheel. Due to the arrangement, the torque, introduced by the electric motor, and the torque, introduced by the steering wheel, are introduced as a sum into the second half of the internal gear wheel of the second planetary gear train.
The second half of the internal gear wheel of the second planetary gear train introduces the torque directly into the non-rotationally connected driven device. The driven device may be a driven shaft or a conversion transmission for converting the rotational movement into a translational movement, for example, a ball-type linear drive.
The arrangement of the stator of the electric motor with the energizing coils, the stator being fixed to the housing, makes it easily possible to couple the electric motor electrically to the vehicle.
Moreover, the arrangement of the steering differential, attached to the body of the car, increases the positional stability between the steering wheel and the power take-off in a structurally simple manner.
Pursuant to the invention introduced, the steering differential can be disposed between the steering gear and the steering wheel, as well as between the steering gear and the steering tie rod. The selection is made in accordance with the respective circumstances of the available space and according to other technical and commercial requirements. In the event that the steering differential is disposed between the steering gear and the steering tie rod, the drive shaft will usually be connected directly with a conversion transmission for converting a rotational movement into a translational movement. For example, a ball-type linear drive is driven directly here.
Advantageously for the further development of the invention, a safety clutch or a circuit, which forces a direct mechanical coupling between the driving shaft and the driven shaft in the event of a fault or of special driving situations, such as a power failure, a computer defect or, when the ignition is switched off, etc., is integrated in the steering differential. The torque, introduced by the electric motor, is then without effect and the driver, due to the mechanical coupling, has complete control of the steering system.
The same coupling may be provided with a different step, for which the drive shaft and, with that, the steering wheel are uncoupled or also locked in position against rotation under pre-tension. However, the driven shaft is controlled by the electric motor by a control device. This last case may be used, for example, for automated parking. In this way, even functions, which otherwise can be represented only with a steer-by-wire system, may be realized.
In an alternative embodiment, the electric motor is disposed parallel to the axis of the steering differential and is coupled over a spur gear or belt drive or chain drive to the conversion transmission or driving transmission.
The inventive transmission may also be operated with a hydraulic driving mechanism, such as an orbital engine or a “reversed” vane-type pump.
As an alternative to using planetary gear trains, it is also possible to use other planetary gearing, such as a harmonic drive transmission. The transmissions may also be constructed as friction drives.